Do Cooling Flows Survive Cluster Mergers?

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Abstract
We report the results of recent numerical simulations of the head-on merger of a cooling flow cluster with an infalling subcluster of galaxies. The objective of these sim- ulations was to examine the effects of different types of cluster mergers (with 16:1 and 4:1 mass ratios) on the evolution of cluster cooling flows (with mass accretion rates of 100 and 400 M⊙/year). The 2-dimensional simulations were performed with a com- bined Hydrodynamics/N-body code on a uniform grid with a resolution of 20 kpc (� 12 zones/core radius). In our simulations, cooling flow disruption is indicated by a dramatic increase (by a factor of 10-40) in the central cooling time of the primary cluster. We find that the ram-pressure of the infalling gas is crucial in determining the fate of the cooling flow as disruption occurs when a substantial amount of subcluster gas reaches the primary's core. In such cases, the subcluster gas can increase the central cooling time by displacing the high-density cooling gas and by heating it via shocks and turbulent gas motions. However, the fate of a merging cooling flow is also dependent on its initial cooling time. In cases where the initial cooling time is very small (i.e., 10-40 times smaller than the Hubble time) then, even if the flow is disrupted, the central cooling time will remain less than a Hubble time and the flow will likely re-establish itself. This has an important observational consequence because such clusters will be classified as cooling flows on the basis of their cooling times even though they have experienced a significant merger. In addition, we find that there is a time delay between core-crossing and the point at which the central cooling time of a disrupted flow becomes of order a Hubble time. Thus, even in the case of disruption, a cluster can be classified as a cooling flow and exhibit substructure (indicative of a merger) for 1-2 Gyr after merging with a subcluster. We argue that our results make it possible to reconcile the high cooling flow frequency